def prost(x):
   k = 0
   for d in range(1, x+1):
          if x % d == 0:
                k += 1
   if k == 2:
      return True
   else:
      return False
maxx = 0
num = int(input())
if num >= 1000 and num <= 9999:
   n1 = str(num)
   for chifra in range(len(n1)):
      if prost(int(n1[chifra])) == True:
         maxx = max(maxx,int(n1[chifra]))
   for chislo in range(len(n1)-1):
      if prost(int(n1[chislo]+n1[chislo + 1])) == True:
         maxx = max(maxx,int(n1[chislo]+n1[chislo + 1]))
   for chislo in range(len(n1)-2):
      if prost(int(n1[chislo]+n1[chislo + 1]+n1[chislo + 2])) == True:
         maxx = max(maxx,int(n1[chislo]+n1[chislo + 1]+n1[chislo + 2]))
   for chislo in range(len(n1)-3):
      if prost(int(n1[chislo]+n1[chislo + 1]+n1[chislo + 3])) == True:
         maxx = max(maxx,int(n1[chislo]+n1[chislo + 1]+n1[chislo + 3]))
   print(maxx)
else:
   print(0)
               
